This invention relates generally to the packaging of articles such as cans, bottles and the like in paperboard trays and more particularly to the packaging of these articles by applying a sheet of film over the articles in the tray and causing the film to bond to the tray by heating the film so as to hold the articles in the tray.
Machines which package articles in paperboard trays by applying a sheet of film over the articles and bonding the film to the tray by heating the film are known in the prior art. Such machines have been produced by Huntingdon Industries of Bethayres, Pa., under the trade name "Wrapcap." In these machines, open top paperboard trays loaded with the articles to be packaged are moved along a prescribed path. As the trays are conveyed along the path, a sheet of thermoplastic film is dispersed over the articles in the tray and wrapped over the articles in the tray by a plurality of flight bars moving along an endless path. The flight bars mount heating units thereon that press the film against opposite sides of the tray and heat the film sufficiently to cause the film to adhere or weld to the tray. After the film is welded to the tray, the film is severed between trays to separate them with the film bonded to the tray holding the articles in the tray to form an integral package. Typically, the film used is heat shrinkable and the thusly formed package is heated so as to shrink the film and tightly lock the articles in the tray.
In these machines, the heating units are continuously heated so that the heating surface on the heating unit coming in contact with the film is maintained at a substantially constant temperature sufficiently high to cause the film to weld or bond to the tray while the heating unit is pressing the film against the tray. The length of time that the film is pressed against the tray is determined by the speed at which the machine is operated since the flight bars move along a fixed endless path. This speed is determined by the rate at which the trays are fed to the flight bars so that the minimum time the film is pressed against the tray occurs when the machine is operating at maximum capacity. Since the welding or bonding of the film to the tray is also dependent on the time that the film is heated while being pressed against the tray, the minimum welding time occurs when the machine is operating at maximum capacity. Thus, the machine is designed so that, when the machine is operating at capacity, the speed of operation is such that the time the film is pressed against the tray is sufficient to effect welding of the film to the tray.
In practice, however, the flow of trays to the packaging machine may be interrupted so that the operational speed of the machine is reduced or stopped. This causes the length of time that the heating units press the film against the tray to be increased. This has created problems in that, if the film is held at welding temperature for too long a period of time, many of such films exhibit an undesirable characteristic of melting or burning through. This is especially true of monolayer films. In an attempt to overcome this burn through problem, special films such as multilayer films which do not exhibit this burn through phenomenon were used in these machines. Since these special films are usually more expensive than those which exhibit burn through characteristics, they thus increased the overall packaging costs of the trays.